Chemistry Reference
In-Depth Information
growth reactions in typical chain-and step-growth polymerizations is often of the
order of 10
4
.
It is normal, then, to use catalysts and elevated temperatures to accelerate
step-growth polymerizations. The activation energies that characterize most such
polymerizations are about 84 kJ/mol. This means that a reaction at 200
Cis300
times as fast as at 100
C and it is 2000 times as rapid at 250
C as at 100
C.
The exothermic enthalpy of polymerization is about
25 kJ/mol for
polyester and polyamide syntheses. This low exotherm makes it possible to carry
out such polymerizations in viscous media at high temperatures without danger of
a runaway reaction due to limitations of heat transfer rates through the reactor
walls. When step-growth reactions are accelerated by application of heat and cata-
lysts, depolymerization reactions also become important. This affects the condi-
tions under which the polymerization can be carried out. To illustrate the basic
principles, we consider a step-growth polymerization in which the initial concen-
trations of reacting groups are equal as in
8.4 to
2
2
HOC
C
OH
+
HO
OH
HOC
CO
OH
+
H
2
O
(12-1)
O
O
O
O
QC
at any time, barring side reactions which could con-
sume one or the other functional group. Then, if
C
0
is the initial concentration and
p
is the extent of reaction (Section 7.4.2) at any instant during the polymerization,
Here [COOH]
Q
[OH]
C
5
C
0
ð
1
2
p
Þ
5
½
COOH
5
½
OH
(12-2)
and
CO
C
o
p
(12-3)
O
since a fraction
p
of the initial
C
0
groups of either kind will now be part of ester
groups. The equilibrium constant
K
for reaction (12-1) is
C
O
H
2
O
(12-4)
O
K =
COOH
OH
or with
Eqs. (12-2) and (12-3)
,
2
K
p
½
H
2
O
=
C
0
ð
1
p
Þ
(12-5)
5
2
To obtain a high polymer in equilibrium step-growth reactions,
p
must be
close to unity (Section 7.4.2). Then, with
p
very close to 1,
2
2
ð
1
2
pÞ
=p
C
ð
1
2
pÞ
5
½
H
2
O
=C
0
K
(12-6)
and the limiting conversion
p
lim, which can be attained under these conditions,
will be given approximately by
